This invention relates in general to a photographic camera having a device for converting an exposure control signal into a mechanical setting of one exposure value factor, such as the setting of the diaphragm aperture, or into a setting of another exposure value factor, such as the time of release of the closing movement of a shutter, thereby determining the duration of the exposure or what is often referred to as the shutter speed.
One aspect of the invention relates to electronic circuit means for producing a first analog signal responsive to the film sensitivity and to the exposure value factors (shutter speed or exposure time, and diaphragm aperture) introduced either manually or automatically in consideration of the brightness of the subject being photographed, and a reversible n-bit counter, a digital-analog converter for delivering a second analog signal corresponding to the respective digital counter output combination, an apparatus for the forward stepping of the counter step by step in a first counting direction, when the second analog signal is smaller than the first, and in the opposite counting direction when the second analog signal is greater than the first, and having a decoder connected to the counter.
Another aspect or feature of the invention relates to the storage of an electric signal produced in analog form, especially an analog signal representing the brightness of the subject to be photographed, in an exposure control circuit for photographic cameras.
Still another feature or aspect of the invention relates to a device for indicating the exposure value in a photographic camera, in which the exposure value is introduced as an analog electric signal, either by hand or by an automatic circuit, the indicating device having a luminous diode line arranged preferably in the viewfinder of the camera, with each luminous diode connected with one output of a decoder actuated by a digital counter.
One known camera of this general kind is disclosed in German offenlegungsschrift (published patent application) no. 2,417,999, published Oct. 24, 1974 which corresponds to U.S. Pat. No. 3,899,788, granted Aug. 12, 1975. In the camera disclosed in this German application, the decoder serves exclusively for the digital indication of the value counted into the counter, while the second analog signal taken from the output of the digital analog converter is used for the exposure time control. This second analog signal is, however, logarithmically compressed, for in order to be able to cover the entire brightness range of a camera with acceptable expense for the control circuit, the signal voltage representative of the subject brightness is always made proportional to the logarithm of the subject brightness. In order that this logarithmic analog signal may be used as an exposure control signal, its anti-logarithm must be found, which takes place in a so-called anti-log circuit preceding the exposure time-control device. However the finding of the anti-logarithm of an analog signal raises several serious problems, since such an anti-log or de-logarithmation circuit, which in the known camera consists of the series connection of a transistor and at least one diode, is intensely temperature-dependent. This has the consequence that in the case of temperature variation the exposure time indication which is carried out through the decoder and which therefore is temperature-independent, indicates a value other than that with which the shutter time is formed. Moreover it requires an expensive temperature-compensation circuit in order that an exact exposure control may be achieved in the temperature range from minus 10.degree. to plus 50.degree. C. required ordinarily by a camera. Such a temperature compensation however can be mastered only with difficulty in the mass production of the camera.
One feature of the present invention is therefore based upon the problem of developing a camera of the stated kind, while avoiding the stated disadvantages, so that the finding of the anti-logarithm of an analog signal for exposure or diaphragm control is avoided, and thus temperature compensation becomes superfluous. Furthermore such a measure is to be carried out with minimum possible expense.